1. Field of the Invention
Embodiments of the invention pertain to charge pump circuits and to circuits and devices incorporating charge pump circuits.
2. Related Technology
Wireless communication devices typically require a frequency synthesis element to produce frequencies for modulating transmitted signals and demodulating received signals. Frequency synthesis is typically provided using a phase locked loop circuit. FIG. 1a shows an example of a conventional 3rd order phase locked loop, and FIG. 1b show an example of a conventional >3rd order phase locked loop. The phase locked loop is a feedback circuit comprised of a phase frequency detector 10, a charge pump 12, a low pass filter 14, a voltage controlled oscillator 16, and a frequency divider 18. The phase frequency detector 10 receives as inputs a reference frequency Fref and an output frequency Fout produced by the voltage controlled oscillator 16. The phase frequency detector 10 compares the phases of the two input signals and generates up and down control signals that are provided to the charge pump 12. The charge pump 12 drives current into or out of the low pass filter 14 in response to the up and down control signals. The output frequency of the voltage controlled oscillator 16 is controlled by the charge stored in the low pass filter 14. The frequency produced by the voltage controlled oscillator 16 is provided as input to the frequency divider 18, which divides the input frequency by an integer n. Consequently, the phase difference detected by the phase frequency detector 10 controls the output frequency Fout of the phase locked loop in response to the input frequency Fref.
An important requirement for communication devices is phase noise. FIG. 2 shows noise levels in the conventional phase locked loop circuits of FIGS. 1a and 1b. As seen in FIG. 2, the conventional circuits produce an out-of-band preference spur having a suppression of approximately 50 dB, which is detectable in the output of the circuit. The preference spur presents a problem for modulation circuits that use higher-order modulation schemes, such as QAM modulation circuits using constellations of 64 or 256 symbols. The conventional circuit also produces an in-band normalized phase noise of approximately −200 dBc/Hz.
It has been determined that the charge pump is a significant source of noise in the phase locked loop circuit. FIG. 3 shows a schematic diagram of a conventional charge pump circuit. The charge pump is comprised of current sources 20, 22 that drive current into and out of an output node 36. The current sources are selectively coupled to the output node 36 by switches 28, 30, thereby controlling the charge that is stored in the low pass filter 14.
In the ideal charge pump, the currents of the current sources 20, 22 are identical. Conventional designs attempt to achieve a current source match of less than 0.1% by implementing the current sources as matched MOS transistors that receive the same control voltage at their gates and that are operated in the non-linear range. However, in practice, variations in supply voltage and in the threshold voltages of the matched transistors tend to produce unequal output currents that may vary by 10% or more. Current mismatch has been identified as a major source of preference spurs.
Scaling of components to small critical dimensions produces further problems in conventional charge pump circuits. The use of 0.18 micron technology in charge pump circuits limits the supply voltage to approximately 1.8 V, and as shown in FIG. 4, the current sources begin to operate in the linear range when the voltage driving the current source falls below approximately 400 mV. This creates additional current mismatch when the voltage at the output node falls below 400 mV, causing further degradation. A conventional solution to this problem is to implement the current sources as transistors having a large ratio of channel width to channel length. However, the use of higher transconductance components introduces more current source noise into the phase locked loop at every phase comparison instant. This degrades of the spectral purity of the phase locked loop. In systems using high-order phase modulation such as wireless LANs, this design may not meet the stringent requirements for low in-band phase noise.
Consequently, conventional charge pump circuit designs have several shortcomings that limit phase locked loop performance, including the production of preference spurs and poor operation at small critical dimensions.